The present invention relates to propulsion systems for water craft and, more particularly, to an improved contra-rotating propeller arrangement.
It has long been recognized that contra-rotating propeller systems have the advantage of eliminating the turning effect or rolling effect of torque produced by the action of a single propeller. If a single propeller is employed, for example, the craft rudder must be used to compensate for the propeller-produced torque. The rudder thus introduces a drag factor which reduces the propulsion efficiency of the engine. This problem becomes more important in high speed water craft wherein the torque produced by a propeller is greater and hence the compensatory effect of the rudder must be greater. Moreover in rough water at high speeds the submersion level of the craft is variable so that the torque balancing effect of a rudder is irregular and can cause oscillatory rolling of the hull. Contra-rotating propellers permit balancing of the torque produced by each propeller, together with the effect of flow past the propeller support struts, and permit attainment of much higher propulsion efficiency factors.
Unfortunately, prior art contra-rotating propulsion systems, such as disclosed in U.S. Pat. No. 3,469,556 to Campbell et al, have suffered from practical problems which have restricted the utilization of these systems, particularly for high speed water craft. One such problem has been mechanical complexity of the propeller drive system, the complexity leading to unacceptable mechanical failure rates.
Another problem with prior art contra-rotating propellers relates to the fact that torque balance is achieved only over a small range of relative propeller speeds. Most contra-rotating propellers are driven by the same engine which limits the propeller speed ratios attainable and thereby limits the flexibility of the propulsion system. In the aforementioned Campbell et al patent it is suggested that the contra-rotating propellers be driven independently by separate engines. This permits torque balancing over a wide variety of conditions; however, the Campbell et al arrangement introduces other limiting factors. For example, Campbell et al mount the contra-rotating propellers on concentric drive shafts. The concentric shaft arrangement provides a larger wetted surface than a single shaft and therefore produces additional drag. Moreover, Campbell et al require that the engines be disposed along opposite sides of the concentric drive shafts, thereby requiring a wider hull than is optimum for high speed craft. Specifically, it is known that the ability of a hull to pass at high speeds over rough water with minimum impact force is closely related to the slenderness ratio of the hull. If machinery considerations dictate hull width, optimum hull design for high speed operation must suffer. Further, since Campbell et al drive the concentric propeller shafts via a common gear box, a single failure in either the gear box or the shaft structure can disable both propellers.
Still another problem associated with prior art contra-rotating propeller systems relates to the fixed physical orientation between the two propellers. That is, whether driven by concentric drive shafts or positioned side-by-side, no prior art contra-rotating propellers are movable relative to one another and to the craft hull. This limitation eliminates a dynamic steering capability wherein a propeller would be pivotable to produce a controllable and dynamic steering force on the craft. In addition, the fixed orientation permits no adjustment of the thrust axis direction which is a valuable capability in craft with planing-type hulls. Moreover, the fixed orientation prevents simple removal of one propeller for low speed operation so that the non-operating propeller does not remain in the water to present unwanted drag.
It is also desirable in most water craft, and particularly in high speed planing hulls to have the center of gravity of the craft as low as possible so that the craft is more stable in the water. In prior art contra-rotating propeller arrangements the locations of the engines and gear boxes, as dictated by the propeller location, is other than optimum.
It is therefore an object of the present invention to provide a contra-rotating propeller system which eliminates all of the aforementioned disadvantages.
It is another object of the present invention to provide a contra-rotating propeller arrangement which is suitable for use in high speed water craft and has a minimal wetted surface area.
It is another object of the present invention to provide a contra-rotating propeller system which has a built-in dynamic steering capability.
It is still another object of the present invention to provide a contra-rotating propeller system in which the supports for the propellers are movable relative to one another.
It is another object of the present invention to provide a contra-rotating propeller system wherein each propeller is independently driven by a separate engine but wherein the propellers are so oriented that the location of the engines does not dictate the width of the hull.
It is yet another object of the present invention to provide a contra-rotating propeller system with completely independent propeller drive systems.
It is another object of the present invention to locate the engines and gear box of a contra-rotating propeller system as low as possible to provide a low center of gravity for the craft.